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The Relativity (and Infinite Improbability?) Drive

It’s here, it’s queer, get used to it. Really it is. Stanley Shawyer, a senior aerospace engineer in England, has built a working prototype, and it is beyond queer. It’s downright magic. He’s calling it a staid-sounding “electromagnetic drive,” or emdrive, but he could have called it the Infinite Improbability Drive. (Hat tip: Douglas Adams, of course).

As far as I can tell, this has not appeared in a peer-reviewed journal (Physical Review Letters, are you asleep at the switch?) so maybe it is a load of bollocks as one scientist quoted in the article below says, but on the other hand, a number of other scientists say it’s brilliant. Time will tell.

The emdrive is based on the force exerted by light when it hits a surface, which is the same force used by solar sails. The difference is that it uses waveguides to channel and amplify the pressure of light. (Waveguides are like fiberoptic strands in one of those novelty table lamps, except that the ends in this case do not allow the light to escape.) The waveguides are shaped so that the photons exert more pressure at one end than they do at the other. Whenever there’s a difference in energy levels, it can be used to do something, whether it’s to push a space craft or to heat a pot of water. But the light is bouncing around inside the sealed waveguide, so how does that exert any pressure outside the “light pipe”?

This is where the magic comes in. Because the photons are travelling at the speed of light, the forces they generate have to be understood in terms of Einstein’s special theory of relativity. And that says things moving at light speed are in their own frame of reference. They’re in their own universe, so to speak, and the asymmetrical force inside the cavity exerts a push as if it was outside of it.

No, I don’t understand it either. But then, I don’t understand gravity, and that doesn’t stop it from exerting force.

Excerpts from the article in New Scientist by Justin Mullins, Sept. 8, 2006. (via Slashdot.)

Take a standard copper waveguide and close off both ends. Now create microwaves using a magnetron, a device found in every microwave oven. If you inject these microwaves into the cavity, the microwaves will bounce from one end of the cavity to the other. According to the principles outlined by Maxwell, this will produce a tiny force on the end walls. Now carefully match the size of the cavity to the wavelength of the microwaves and you create a chamber in which the microwaves resonate, allowing it to store large amounts of energy.

What’s crucial here is the Q-value of the cavity – a measure of how well a vibrating system prevents its energy dissipating into heat, or how slowly the oscillations are damped down. For example, a pendulum swinging in air would have a high Q, while a pendulum immersed in oil would have a low one. If microwaves leak out of the cavity, the Q will be low. A cavity with a high Q-value can store large amounts of microwave energy with few losses, and this means the radiation will exert relatively large forces on the ends of the cavity. You might think the forces on the end walls will cancel each other out, but Shawyer worked out that with a suitably shaped resonant cavity, wider at one end than the other, the radiation pressure exerted by the microwaves at the wide end would be higher than at the narrow one.

… The result is a net force that pushes the cavity in one direction.

And the device seems to work: by mounting it on a sensitive balance, he has shown that it generates about 16 millinewtons of thrust, using 1 kilowatt of electrical power. Shawyer calculated that his first prototype had a Q of 5900. With his second thruster, he managed to raise the Q to 50,000 allowing it to generate a force of about 300 millinewtons – 100 times what Cosmos 1 could achieve. It’s not enough for Earth-based use, but it’s revolutionary for spacecraft.

Shawyer is looking ahead to the next stage of his project. He wants to make the thrusters so powerful that they could make combustion engines obsolete, and that means addressing the big problem with conventional microwave cavities – the amount of energy they leak. The biggest losses come from currents induced in the metal walls by the microwaves, which generate heat when they encounter electrical resistance. This uses up energy stored in the cavity, reduces the Q, and the thrust generated by the engine drops.

Fortunately particle accelerators use microwave cavities too, so physicists have done a lot of work on reducing Q losses inside them. The key, says Shawyer, is to make the cavity superconducting. Without electrical resistance, currents in the cavity walls will not generate heat. Engineers in Germany working on the next generation of particle accelerators have achieved a Q of several billion using superconducting cavities. If Shawyer can match that performance, he calculates that the thrust from a microwave engine could be as high as 30,000 newtons per kilowatt – enough to lift a large car.

This raises another question. Why haven’t physicists stumbled across the effect before? They have, says Shawyer, and they design their cavities to counter it. The forces inside the latest accelerator cavities are so large that they stretch the chambers like plasticine. To counteract this, engineers use piezoelectric actuators to squeeze the cavities back into shape. “I doubt they’ve ever thought of turning the force to other uses,” he says. [This reminds me of the (apocryphal) story about Robert Boyle and his buddies in the 1600s who realized that the steam pushing up a pot lid could be used to push anything.]

Then there is the issue of acceleration. Shawyer has calculated that as soon as the thruster starts to move, it will use up energy stored in the cavity, draining energy faster than it can be replaced. So while the thrust of a motionless emdrive is high, the faster the engine moves, the more the thrust falls. Shawyer now reckons the emdrive will be better suited to powering vehicles that hover rather than accelerate rapidly. A fan or turbine attached to the back of the vehicle could then be used to move it forward without friction. He hopes to demonstrate his first superconducting thruster within two years.

Be great if it worked out on schedule, but I’m willing to wait three, even four years for my levitating car.

Technorati tags: relativity drive, electromagnetic drive, emdrive, Shawyer, science, energy, technology, space